1. Field of the Disclosure
This disclosure generally relates to an image sensor and, more particularly, to an image sensor utilizing the analog differencing and an operating method thereof.
2. Description of the Related Art
A conventional image sensor generally has a plurality of sensing pixels arranged in array, wherein one operating method of the image sensor, for example, uses a rolling shutter to successively activate, with a control signal, each row of the sensing pixels in the image sensor to capture an image. Since each row of the sensing pixels is successively activated instead of simultaneously activated (i.e. start timings for each row of the sensing pixels to be exposed are different), the image captured by using the rolling shutter may have distortion in capturing an image of a fast moving object.
Another operating method of the image sensor uses a global shutter, for example, to simultaneously activate all sensing pixels in the image sensor with a control signal so that each row of the sensing pixels starts to be exposed at an identical time to capture an image. Therefore, the image sensor using the global shutter may avoid the problem of distortion.
However, in order to eliminate ambient light interference or reduce the image noise when the image sensor uses the global shutter to capture an image, one conventional solution is to directly perform a subtraction between two digital image frames through image post processing so as to obtain a differential image frame. For example, referring to FIGS. 1A and 1B, FIG. 1A is a timing diagram of a conventional image sensor 91 using the global shutter to capture an image. It is assumed that the image sensor 91 has 4 rows of sensing pixels R1-R4. In a first period P1, a light source is turned on for a predetermined time and the rows of sensing pixels R1-R4 are simultaneously exposed. Then, the light source is turned off and the rows of sensing pixels R1-R4 are successively read to output a first image signal. In a second period P2, the light source is turned off and the rows of sensing pixels R1-R4 are simultaneously exposed for the predetermined time, and the rows of sensing pixels R1-R4 are successively read to output a second image signal.
Then, referring to FIG. 1B, the first image signal outputted from the image sensor 91 is firstly converted into a first digital signal 9a through an analog to digital converter 93 to be stored in a digital buffer 95. Then, the second image signal is converted into a second digital signal 9b through the analog to digital converter 93. Finally, the second digital signal 9b is subtracted from the first digital signal 9a to obtain a third digital signal 9c in which ambient light is removed. However, in this way, the digital buffer 95 has to be disposed in a system including the image sensor 91 and the image sensor 91 must successively output two image frames (e.g., image frames formed according to the first digital signal 9a and the second digital signal 9b) so that one processed image frame is obtainable (e.g., an image frame formed according to the third digital signal 9c).